Noise 04 Expedition when a/c or heater on

I don't know what it is but it comes on when the air is turned on inside the truck. Heat or A/C doesn't matter and its coming from rear passenger side by the window under the plastic molding. I thought it was a secondary blend door but as I know it the truck only has one. Im really skeptical and would appreciate some guidance. Thanks in advance.

Is the sound like a owl hoot and has about 30 seconds between hoots? Or is it a different sound?